A liquid crystal display (“LCD”) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It has many advantages over competing technologies because it uses very small amounts of electric power and is therefore suitable for use in battery-powered electronic devices, and because of its thinness.
Each pixel in an LCD consists of a layer of liquid crystal (“LC”) molecules suspended between two transparent electrodes, and sandwiched between two crossed linear polarizers (i.e., polarizers with axes of transmission which are perpendicular to each other). Without the liquid crystals between them, light passing through one polarizer would be blocked by the other. The liquid crystals act as polarization modifying light valves by changing the polarization state of the light coming from the rear polarizer. In order to function in this manner, the liquid crystal molecules must be correctly aligned so that they accept light of the polarization state transmitted by the rear polarizer and can rotate it to the polarization state that is transmitted by the front polarizer. Various techniques are known for achieving the appropriate alignment of the liquid crystal molecules. These include mechanical rubbing, which introduces microscopic grooves, or use of oriented linearly polarized UV illumination of an appropriate alignment layer substrate. Application of an electric field, by applying a voltage to the transparent electrodes, can modify the degree of polarization rotation, thus enabling fine control of the light passing through the pixel. The operation of LCDs depends on the correct relationship between the axis of transmission of the rear polarizer, the alignment of the liquid crystal layer (both for light entering and leaving) and the axis of transmission of the front polarizer.
The pixels by themselves do not generate light and therefore an LCD requires external illumination, either from ambient sources for a “reflective” LCD or from a backlight for a “transmissive” LCD (or from a combination in the case of a “transflective” LCD). A typical transmissive LCD has a backlight unit that consists of a light source and a light guide. The light guide is used to transform the light output by the light source into an even distribution across the LCD panel (typically the light enters the light guide and is constrained to continue propagating within it by total internal reflection, until it encounters an extraction feature). A large portion of the power consumption of a transmissive LCD is devoted to the backlight. However one problem with known transmissive LCDs is that the majority of this power is expended in producing light that is ultimately not used in the display output, since it is filtered out. A typical light yield (i.e., the fraction of generated light that is transmitted by a fully active pixel) of these known LCDs is approximately 5%-7%.
Light loss that is intrinsic to LCD designs is typically due to the following elements (assuming illumination from an unpolarized white source):
color filter set: approximately 28% transmission;
aperture ratio: approximately 70% transmission; and
rear and front polarizers: approximately 40% transmission.
Color filters are required since backlights typically generate white light. The aperture ratio arises since some of the area of an LCD does not transmit light.
Polarization losses arise from intrinsic aspects of the design of LCDs. As has been described, LCDs require illumination to be linearly polarized and appropriately oriented, which typically results in the loss of at least half of the light available from the backlight.
Various attempts have been made to improve the light yield of LCDs, which could greatly improve the electrical efficiency of LCDs, therefore enabling more power efficient appliances, extending battery life for mobile devices, reducing needed backlight illumination components since fewer or lower power lamp elements would be needed to provide a certain level of brightness, and improving heat management in display units since much of the lost light is absorbed as heat.
One known prior art approach for improving light yield involves the use of a Polarization Separation Element (“PSE”) and a Polarization Conversion Element (“PCE”). For example U.S. Pat. No. 7,038,745 discloses the use of a reflective rear polarizer, which transmits a first polarization state and reflects a second polarization state, as a PSE and a Lambertian rear reflector in the backlight unit as a PCE. The Lambertian rear reflector depolarizes the second polarization to include some portion of the first polarization state, which can then be transmitted by the reflective rear polarizer, thereby improving light yield. In this disclosed approach, the polarization separation occurs after unpolarized light has been extracted from the light guide by frustrated total internal reflection. Commercial versions of this technology are in use and though it represents an improvement over previous approaches, a significant portion of the output of the light source is still not utilized.
Another known approach for improving light yield is disclosed, for example, in U.S. Pat. Application 2007/0064445, which discloses the use of a polarization beam splitter (“PBS”) between the light source and the light guide as a PSE where the non transmitted output from the PBS is directed to a wave plate that operates as a PCE, the output of which feeds into the light guide. In this approach, polarization separation occurs before light enters the light guide. One problem with this approach is the efficiency of conversion of the PCE and its variation of performance by incident angle and wavelength, which leaves a portion of the output of the light source not utilized.
A further prior art approach is disclosed in U.S. Pat. No. 6,234,639 which discloses polarization selective out-coupling as a PSE and a quarter-wave plate and reflector as a PCE. Unpolarized light is coupled into a light guide which has polarization selective out-coupling based on reflection at a series of inclined surfaces within the light guide, which have appropriately tuned variations in refractive indices. These features selectively extract a first polarization state and continue the Total Internal Reflection (“TIR”) propagation of a second polarization state, which eventually encounters the PCE, where some is converted to the extractable polarization state. As disclosed, polarization separation occurs in the light guide during the process of extraction. Again the efficiency of the conversion process leaves a portion of the output of the light source not utilized.
Several other polarization selective out-coupling techniques are known and have been applied to light guides. For example, U.S. Pat. No. 6,796,669 discloses the use of inclined reflective films; U.S. Pat. No. 6,750,996 discloses the use of a volume hologram, which is equivalent to a stack of inclined planes with varying refractive indices; U.S. Pat. No. 7,072,544 discloses the use of Polymer Dispersed Liquid Crystal (“PDLC”) where the dispersed phase has anisotropic refractive indices; and U.S. Pat. App. 2003/0058383 discloses the use of a birefringent micro-structured layer. Some of these polarization selective out-coupling approaches benefit from extracting light in a confined range of directions, which can be useful in minimizing the need for prism films or other output collimation structures.
Based on the foregoing, there is a need for an LCD system that has an improved light yield relative to known systems.